1. Field of the Invention
The present invention relates to an infrared optical element and an anti-reflection film used therein.
2. Description Related to the Prior Art
An infrared camera that captures an image using infrared radiation is widely available in recent years. As is widely known, the infrared camera is suitable for imaging in a dark place or through a screen i.e. a thin sheet of cloth. On account of this characteristic, the infrared camera is often set up in various facilities such as a store and a shop as a security or surveillance camera for crime or disaster prevention. The infrared camera is also used in measurement of water volume of a dam, measurement of traffic, and the like. Furthermore, a safe driving support system using the infrared camera is recently known. In this system, the infrared camera is provided on a car body, and a live image captured by the infrared camera is displayed on a monitor installed in a car. This system expands vision of a driver at nighttime, as compared with vision only with visible light, to support safe driving.
A high percentage of infrared radiation is absorbed by atmospheric components including water vapor and the like. Thus, the infrared camera uses infrared radiation of a wavelength band limited within a so-called infrared atmospheric window, in which the absorption of the infrared radiation does not occur. Out of such wavelength band, a near-infrared camera that uses near-infrared radiation having wavelengths of 0.7 to 2.5 μm and a far-infrared camera that uses far-infrared radiation having wavelengths of 3.0 to 1000 μm are in practical use, for example.
Since the infrared camera uses light with long wavelengths, which is categorized as infrared radiation, an optical element e.g. a lens of the infrared camera is composed of a base made of infrared transmitting glass with high refractivity. As conventional infrared transmitting glass, for example, germanium (Ge), zinc selenide (ZnSe), and the like are known. The infrared transmitting glass described above, however, is expensive and has a cost disadvantage. Furthermore, since the infrared transmitting glass described above has a crystalline structure, a processing method thereof is limited to polishing. This results in difficulty in processing into a complex configuration such as a microlens array.
Against this backdrop, a new type of infrared transmitting glass (hereinafter called chalcogenide glass) that predominantly contains sulfur (S), selenium (Se), or tellurium (Te) is recently known (U.S. Patent Application Publication No. 2010/285946). The chalcogenide glass is less expensive than a germanium crystal, the zinc selenide, and the like. Also, the chalcogenide glass is easily processed by molding into the base of the optical element such as the lens, a prism, and a filter.
The infrared transmitting glass generally has high surface reflectance because of the high refractivity, and its transmittance is 60 to 70% at most. Thus, it is known that just processing of the infrared transmitting glass into a desired outside shape of the lens or the like is not enough to obtain incident light intensity sufficient for imaging. For this reason, an anti-reflection film is provided on a surface of the base made of the infrared transmitting glass, in order to reduce a loss of the incident light intensity due to surface reflection.
When the germanium or zinc selenide is used as the infrared transmitting glass, there is known an example of an anti-reflection film including layers of ZnS, CaF2, YF3, Y2O3, Ge, Si, and the like. Also, it is known to use TiO2 or Al2O3 as a primary layer to improve adhesion of the anti-reflection film to the base (Japanese Patent Laid-Open Publication No. 6-313802). In the case of using the chalcogenide glass as the base, there are known an anti-reflection film composed of four layers of Al2O3, Ge, ZnS, and MgF2 to improve durability (Japanese Patent No. 3361621), and an anti-reflection film having a layer of CeO2 over layers of Ge, ZnS, CeF3, and the like to improve weather resistance (Japanese Patent Laid-Open Publication No. 2006-72031).
As described above, the infrared camera is set up not only inside a room where an ambient environment is relatively stable, but also outside the room with large fluctuation in temperature, humidity, and the like and even in a hostile environment such as the car body. Accordingly, sufficient weather resistance is required of the anti-reflection film provided on the surface of the base made of the infrared transmitting glass, such that tarnish, exfoliation, cracking and the like do not occur even if the infrared camera is used in the hostile environment. Furthermore, use of an expensive crystalline base made of the germanium or the like causes cost increase, in the optical element of the infrared camera. Therefore, it is desirable that the optical element is composed of an inexpensive base made of the chalcogenide glass.
However, if a conventional anti-reflection film designed for the germanium crystalline base is provided on a surface of the chalcogenide glass base, adhesion of the anti-reflection film is impaired. Thus, the anti-reflection film may fall off in the hostile environment as described above.
Also, the anti-reflection film designed for the germanium crystalline base often contains a layer made of an expensive material such as germanium. Thus, provision of the anti-reflection film that is designed for the germanium crystalline base on the chalcogenide glass base causes cost increase, even with the use of the inexpensive chalcogenide glass base. Also, cost is generally increased with increase in the number of layers of the anti-reflection film, and hence it is desirable that the anti-reflection film is made of only the most inexpensive possible materials and from a fewest possible number of layers.
Furthermore, the anti-reflection film designed for the germanium crystalline base sometimes contains a layer of MgF2. However, the MgF2 does not have sufficient weather resistance. Accordingly, if such anti-reflection film designed for the germanium crystalline base is used as an anti-reflection film of the chalcogenide glass base, the insufficiency of the weather resistance occurs as a matter of course. In a case where an anti-reflection film contains a layer made of a material of low weather resistance such as the MgF2, it is conceivable to provide a layer of high weather resistance at the topmost of the anti-reflection film. However, when only the topmost layer is made of a high weather-resistant material while an inside layer is made of a low weather-resistant material, as described above, if fluctuation in temperature or humidity brings about slight cracking, exfoliation, or the like of the anti-reflection film, the layer of low weather resistance is exposed to the outside. Thus, the cracking or exfoliation rapidly expands from an exposed point, and a life of a product becomes short. For this reason, it is desirable that the anti-reflection film is made only of high weather-resistant materials.